Reflection LCD with counter-reflector having openings at intersection areas of bus lines

ABSTRACT

A reflection type liquid crystal display comprising an active matrix board having active matrix elements at the intersections of signal lines formed of a transparent conducting film and scanning lines formed of a metal, a counter electrode board having a reflector layer thereon, and a liquid crystal sealed between the active matrix board and the counter electrode board which are bonded so that the film-bearing surfaces thereof face each other, wherein the reflector layer is not formed in those parts of the counter electrode board which correspond to the signal lines formed of the transparent conducting film and disposed on the active matrix board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to liquid crystal displays and, moreparticularly, to the improvement of image quality in reflection typeliquid crystal displays using no back light.

2. Description of the Related Art

Liquid crystal displays are devices in which a liquid crystal layerhaving a thickness of the order of 5 μm is sandwiched between two glassplates having electrodes thereon and images are displayed by controllingthe movement of the liquid crystal molecules. Accordingly, they canprovide much thinner display units than CRTs.

In a common TN liquid crystal display, images are displayed by disposinga back light source on the outside of the liquid crystal display andcontrolling the liquid crystal molecules so as to transmit or absorblight from the back light source. This is called a transmission typeliquid crystal display. The transmission type liquid crystal display hasthe disadvantage that the use of a back light source causes an increasein power consumption.

One means for solving this problem is provided by a reflection typeliquid crystal display in which external light is introduced andreflected instead of using back light. When a reflector is disposed onthe outside of a TN liquid crystal display, this liquid crystal displayhas the disadvantages that the displayed image is generally dark and thedisplayed characters or the like produce shadows. The reason for thedarkening is that the TN liquid crystal display requires polarizers and,therefore, the amount of external light introduced is reduced to onehalf. The reason why the displayed characters or the like produceshadows is that, since a reflector is disposed on the outside, there isa great distance between the display screen and the reflective surfaceand, therefore, the display screen is mirrored in the reflector.

A liquid crystal display of the phase transition guest-host (GH) typehas low contrast and is seldom used as a transmission type liquidcrystal display using back light. However, when this is used as areflection type liquid crystal display, it is more disadvantageous thanthe TN liquid crystal display. That is, since no polarizer is required,the amount of light introduced is greater and the displayed imagebecomes brighter. Moreover, since a reflector may be formed within theliquid crystal layer, the displayed characters or the like produce noshadow.

The operating principle of a liquid crystal display of the GH type isdescribed with reference to the schematic section of FIG. 7. As shown inFIG. 7, this liquid crystal display includes, on the panel surface side,a glass substrate 1 on which an ITO layer 2 and a polyimide layer 5serving as a protective layer are formed in that order. On the opposedboard side, it also includes a glass substrate 1 having a reflectorlayer 6 and a polyimide layer 5 formed thereon. A GH liquid crystallayer 7 is sandwiched therebetween to construct the liquid crystaldisplay. In this GH liquid crystal layer 7, several percent of dichroicdye molecules 3 are mixed with liquid crystal molecules 4. Thesedichroic dye molecules 3 do not change their direction under theinfluence of an electric field. However, since dichroic dye molecules 3have a size almost equal to that of liquid crystal molecules 4, theybehave in the same manner as the large number of liquid crystalmolecules 4. Moreover, dichroic dye molecules 3 transmit or absorb lightaccording to their direction. That is, in the absence of an appliedvoltage as shown in FIG. 7(a), dichroic dye molecules 3 are horizontallyoriented, and exhibit a spiral configuration in the presence of a chiraldopant. If the angle of twist is greater than 90 degrees, any type ofpolarized light 19 entering the panel from its surroundings is absorbed,so that the panel looks black. On the other hand, in the presence of anapplied voltage as shown in FIG. 7(b), dichroic dye molecules 3 arevertically oriented in conformity with the orientation of the liquidcrystal molecules. Consequently, incident light is transmitted by GHliquid crystal layer 7, is reflected by the upper reflector layer 6, andexits as outgoing light 20, so that the panel looks white.

As reflector layer 6, there is commonly used a metal film formed bydepositing a metal having high reflectivity (e.g., aluminum or silver)on substrate 1 by sputtering or vapor deposition.

The circuit of an active matrix-driven liquid crystal display actuallyused to display images is described with reference to FIG. 8. In FIG.8(a), image signals are applied to signal lines X1, X2, . . . , Xn. Ateach of the intersections of these signal lines and scanning lines Y1,Y2, . . . , Yn, a thin-film transistor (TFT) 12 is connected. This TFT12 is connected to a pixel electrode, so that the pixel electrode andthe counter electrode, together with a liquid crystal sandwichedtherebetween, constitute a pixel electrode capacitance 13. As shown inFIG. 8(b), drive pulses Z1, Z2, . . . , Zn are successively applied toscanning lines Y1, Y2, . . . , Yn. Taking scanning line Y1 as anexample, TFTs 12 connected to scanning line Y1 are in the conductingstate during a period in which drive pulse Z1 has a voltage of 20 V. Asa result, the electric potential of the image signals applied to thesignal lines is written into pixel electrode capacitance 13 constitutedby the pixel electrodes and counter electrode 14 with the liquid crystalsandwiched therebetween. The period in which drive pulse Z1 has avoltage of 20 V is equal to 1/60 n second which is obtained by dividingthe screen rewriting time, i.e., 1/60 second by the number (n) of thescanning lines. Next, TFTs 12 connected to scanning line Y1 are in thenon-conducting state during a period in which drive pulse Z1 has avoltage of 0 V. Thus, the electric potentials of the image signalswritten into pixel capacitance 13 are retained until the voltage ofdrive pulse Z1 is increased to 20 V in the next scan. In this manner, animage is displayed. TFTs of the type commonly used in activematrix-driven liquid crystal displays are forward-staggered TFTs. Theseforward-staggered TFTs can be manufactured by two patterning steps andare frequently used because of their ease of manufacture.

A process for fabricating an active matrix-driven liquid crystal displayby using these TFTs is described below. First of all, the method ofmaking a TFT board is explained with reference to FIG. 9.

A glass substrate 1 is provided as shown in FIG. 9(a) and an ITO layer 2is deposited thereon by means of a sputtering apparatus as shown in FIG.9(b). A resist is applied thereto, exposed to light with a suitableexposure apparatus through a mask having patterns for defining signallines and pixel electrodes, and then developed to leave the resisthaving, for example, the patterns of signal line X and pixel electrode8. Next, ITO layer 2 is etched by using the patterned resist as a maskto form signal line X and pixel electrode 8. Thereafter, the resist isstripped off as shown in FIG. 9(c). Subsequently, after an ohmic contactlayer is formed only on the patterned regions of ITO layer 2 by plasmatreatment in a plasma CVD apparatus, an amorphous silicon layer 9 and asilicon nitride layer 10 are deposited thereon by means of a plasma CVDapparatus and a chromium layer 11 is deposited thereon by means of asputtering apparatus as shown in FIG. 9(d). A resist is applied thereto,exposed to light with an exposure apparatus through a mask havingpatterns for defining scanning lines, and then developed to leave theresist having the patterns of the scanning lines. Next, chromium layer11, silicon nitride layer 10 and amorphous silicon layer 9 are etched byusing the patterned resist as a mask, and the resist is then strippedoff [FIG. 9(e)]. Finally, a polyimide is applied with a printer and thenrubbed to obtain a TFT board as shown in FIG. 9(f).

Next, the method of making a counter electrode board is illustrated inFIG. 10. A glass substrate 1 is provided [FIG. 10(a)] and a reflectorlayer 6 consisting of aluminum or silver having high reflectivity isdeposited thereon by means of a sputtering apparatus [FIG. 10(b)].Subsequently, a polyimide is applied with a printer and then rubbed toobtain a counter electrode board as shown in FIG. 10(c).

Then, the TFT board and the counter electrode board which have been madein the above-described manner are bonded with a predetermined gaptherebetween. First of all, a sealant is printed on the film-bearingsurface of the TFT board by means of a seal printer. This sealantfunctions as an adhesive for bonding the two boards. In order to securea predetermined gap between the two boards, a gapping material inparticulate form is incorporated in the sealant. Then, the TFT board andthe counter electrode board are bonded with the sealant so that thefilm-bearing surfaces thereof face each other.

Finally, using a liquid crystal injector, a GH liquid crystal isinjected into the gap between the two boards so bonded to fabricate anactive matrix-driven reflection type liquid crystal display. A plan ofthis liquid crystal display is shown in FIG. 11. As shown in thisfigure, signal lines X1, X2, . . . , Xn and scanning lines Y1, Y2, . . ., Yn are disposed crosswise on the TFT board. TFTs 12 are formed in thevicinity of the respective intersections of the signal lines and thescanning lines. A schematic section taken along line A-B in FIG. 11 isshown in FIG. 12, and a schematic section taken along line C-D in FIG.11 is shown in FIG. 13.

In this active matrix-driven reflection type liquid crystal display, anarbitrarily chosen voltage is always applied to the signal lines and thescanning lines, so that the portions of the liquid crystal above thesignal lines and the scanning lines are in the light-transmitting state.However, as shown in FIG. 12, chromium layer 11 forming the scanninglines does not transmit light and, therefore, light 19 striking on thisregion does not pass through chromium layer 11. In contrast, as shown inFIG. 13, ITO layer 2 forming the signal lines and the pixel electrodesis transparent and hence transmits light. Consequently, light istransmitted not only in the region of pixel electrode B to which avoltage is applied, but also in the regions of signal lines X1 and X2.This light is reflected by the counter electrode board and exits to theoutside (as indicated by arrows 21).

In conventional panel structures, an arbitrarily chosen voltage isalways applied to transparent electrodes forming signal lines, asdescribed above. Consequently, the portions of the liquid crystals areoriented under the influence of the resulting electric field and are ina partially transmitting state. The light striking on these regions isreflected by a reflector layer and exits to the outside. That is, areduction in contrast due to the leakage of light through the signallines has posed a problem. Moreover, when a white spot is displayed onthe screen by switching on, for example, the TFT formed in the vicinityof the intersection of signal line X2 and scanning line Y2, as shown inFIG. 14, a white stripe is displayed in the region of the signal linewhich is also in the transmitting state, and makes the image difficultto see. The occurrence of such a phenomenon (what is so-called "crosstalk") has also posed a problem.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention is to prevent theleakage of light through the regions of signal lines formed oftransparent electrodes.

According to the present invention, there is provided a reflection typeliquid crystal display comprising an active matrix board having activematrix elements at the intersections of signal lines formed of atransparent conducting film and scanning lines formed of a metal, acounter electrode board having a reflector layer thereon, and a liquidcrystal sealed between the active matrix board and the counter electrodeboard which are bonded so that the film-bearing surfaces thereof faceeach other, characterized in that the reflector layer is not formed inthose parts of the counter electrode board which correspond to thesignal lines formed of the transparent conducting film and disposed onthe active matrix board.

In the liquid crystal display constructed in the above-described manner,the counter electrode also serving as a reflector layer does not existabove the signal lines formed of the transparent conducting film.Accordingly, an electric field is scarcely applied to the portions ofthe liquid crystal which exist above the signal lines, and the lightentering through the regions of the signal lines is not reflected, sothat these portions never pass into the transmitting state.Consequently, the liquid crystal above the signal lines always remainsin the non-transmitting state, regardless of the voltages applied to thesignal lines. This makes it possible to prevent the disadvantages ofconventional panel structures, i.e., a reduction in contrast which iscaused by the light-transmitting state of the liquid crystal above thesignal lines, and the phenomenon ("cross talk") in which, whencharacters or the like are displayed on the screen, vertical stripes areproduced in the inherently dark background and makes the characters orthe like difficult to see.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan of a TFT board in accordance with the presentinvention.

FIG. 2 is a section taken along line E-F in FIG. 1, showing a firstexample of the present invention.

In FIGS. 3, (a) to (d) show sections illustrating a series of steps inthe method of making a counter electrode board in accordance with thefirst example of the present invention.

FIG. 4 is a section taken along line E-F in FIG. 1, showing a secondexample of the present invention.

In FIGS. 5, (a) to (e) show sections illustrating a series of steps inthe method of making a counter electrode board in accordance with thesecond example of the present invention.

FIG. 6 is a section for explaining the occurrence of disclination.

FIG. 7 includes sections for explaining the operating principle of aconventional liquid crystal display of the phase transition GH type, (a)showing its state in the absence of an applied voltage and (b) showingits state in the presence of an applied voltage.

FIG. 8 includes an equivalent circuit diagram (a) of a conventionalactive matrix-driven liquid crystal display, and a diagram (b) showingdrive pulses applied successively to scanning lines.

In FIGS. 9, (a) to (f) show sections illustrating a series of steps inthe method of making a TFT board.

In FIGS. 10, (a) to (c) show sections illustrating a series of steps inthe method of making a conventional counter electrode board.

FIG. 11 is a plan of a conventional active matrix-driven liquid crystaldisplay.

FIG. 12 is a section taken along line A-B in FIG. 11.

FIG. 13 is a section taken along line C-D in FIG. 11.

FIG. 14 is a plan showing the occurrence of cross talk in a conventionalactive matrix-driven liquid crystal display.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is more specifically explained with reference tothe following examples. However, these examples are not to be construedto limit the scope of the invention.

EXAMPLE 1

Example 1 of the present invention is described with reference to FIG. 1showing a schematic plan and FIG. 2 showing a section taken along lineE-F in FIG. 1.

Referring to FIG. 1, signal lines X1, X2 and X3 and scanning lines Y1,Y2 and Y3 are disposed crosswise on a TFT board. TFTs 12 are disposed inthe vicinity of the respective intersections of the signal lines and thescanning lines. In a reflector layer 6 (hatched area) deposited on acounter electrode board, only the parts located above the signal linesare removed to form slits 17.

Next, the operation of this liquid crystal device is explained withreference to FIG. 2. In FIG. 2, signal lines X1 and X2 and pixelelectrodes A and B are formed by patterning a transparent conductingfilm 2 deposited on a glass substrate 1. Moreover, a polyimide layer 5serving as a protective layer is formed thereon to construct a TFTboard. Furthermore, a GH liquid crystal layer 7 is interposed betweenthis TFT board and a counter electrode board comprising a glasssubstrate 1 having a reflector layer 6 deposited thereon to construct aliquid crystal panel. In reflector layer 6 deposited on the counterelectrode board, only the parts lying opposite to the signal lines onthe TFT board are removed to form slits 17.

The portion of GH liquid crystal layer 7 above pixel electrode Apresents a state in which there is no potential difference betweenreflector layer 6 and pixel electrode A. In this state, incident light19 is absorbed by the GH liquid crystal layer, so that this region ofthe panel looks black. On the other hand, the portion of GH liquidcrystal layer 7 above pixel electrode B presents a state in which thereis a potential difference between reflector layer 6 and pixel electrodeB. In this state, incident light 19 is transmitted by GH liquid crystallayer 7, is reflected by reflector layer 6, and exits as outgoing light20, so that this region of the panel looks white.

Since reflector layer 6 does not exist above signal lines X1 and X2, theportions of GH liquid crystal layer 7 above signal lines X1 and X2 arein a state in which there is no potential difference between signallines X1 or X2 and reflector layer 6. In this state, incident light 19is absorbed by the GH liquid crystal layer, so that these regions of thepanel look black. Even if the electric potentials of the signal linesare changed, no potential difference is produced because reflector layer6 also serving as a counter electrode does not exist above the signallines. As a result, these regions of the panel always look black.

This makes it possible to prevent the shortcomings observed inconventional panel structures, i.e., a reduction in contrast caused bythe light-transmitting state of the liquid crystal above signal lines,and the occurrence of cross talk as described with reference to FIG. 14.Moreover, the white-to-black contrast is improved from the conventionalvalue of 1:3 to 1:5.

Now, the process for fabricating the above-described liquid crystaldisplay is described below.

The method of making the TFT board is the same as known in the priorart. That is, as described previously, the TFT board is-made accordingto the steps shown in FIG. 9.

The method of making the counter electrode board is explained withreference to FIG. 3. First of all, a glass substrate 1 is provided asshown in FIG. 3(a) and a film of aluminum or silver having highreflectivity is deposited thereon by means of a sputtering apparatus asshown in FIG. 3(b). A resist is applied thereto, exposed to light withan exposure apparatus through a mask having a pattern in which the partscorresponding to the signal lines patterned on the TFT board are removedin the form of slits, and then developed to leave the resist having theaforesaid pattern. Next, the metal film is etched by using the patternedresist as a mask to remove the parts of the metal film which are notcovered with the mask. Thereafter, the resist is stripped off as shownin FIG. 3(c). Finally, a polyimide is applied with a printer and thenrubbed to complete the counter electrode board as shown in FIG. 3(d)Then, the TFT board and the counter electrode board which have been madein the above-described manner are bonded with a predetermined gaptherebetween. First of all, a sealant is printed on the film-bearingsurface of the TFT board by means of a seal printer. In order to securea predetermined gap between the two boards, a gapping material inparticulate form is incorporated in the sealant. Then, the TFT board andthe counter electrode board are bonded with the sealant so that thefilm-bearing surfaces thereof face each other. In this step, the twoboards are disposed in such a way that, when viewed from the outside ofthe panel, the patterns of the signal lines on the TFT board are alignedwith the slits on the counter electrode board. Finally, using a liquidcrystal injector, a GH liquid crystal is injected into the gap betweenthe two boards so bonded. Thus, an active matrix-driven reflection typeliquid crystal display as illustrated in FIGS. 1 and 2 is fabricated.

EXAMPLE 2

Referring to FIG. 6, in the liquid crystal display of Example 1, thereis an electric field between pixel electrode B and reflector layer 6.Consequently, a disclination line 16 comprising a discontinuity part ofthe orientation may be produced in the GH liquid crystal layer owing toa transverse electric field created between pixel electrode B and theend of reflector layer 6, and a transverse electric field createdbetween scanning line X2 and the end of reflector layer 6. In this case,light may leak through this region and produce an image of poor quality.

In the liquid crystal display of Example 2, reflector layer 6 is whollycovered with an ITO layer 2 constituting a transparent conducting layer,so that the creation of a transverse electric field is prevented. Now,the operation of this liquid crystal device is explained below. Theportion of GH liquid crystal layer 7 above pixel electrode A presents astate in which there is no potential difference between reflector layer6 and pixel electrode A. In this state, incident light 19 is absorbed byGH liquid crystal layer 7, so that this region of the panel looks black.On the other hand, the portion of GH liquid crystal layer 7 above pixelelectrode B presents a state in which there is a potential differencebetween reflector layer 6 and pixel electrode B. In this state, incidentlight 19 is transmitted by GH liquid crystal layer 7. However, sincereflector layer 6 does not exist on the regions of the counter electrodeboard which lie opposite to signal lines X1 and X2, incident light 19passes through the entire panel. As a result, these regions of the panellook black.

In this example, no transverse electric field is created betweenscanning line X2 and the end of reflector layer 6. Consequently, adisclination line 16 comprising a discontinuity part of the orientationis not produced in the GH liquid crystal layer even under the influenceof a transverse electric field created between pixel electrode B and theend of reflector layer 6 as described above with reference to FIG. 6.Thus, the problem that light may leak through this region and produce animage of poor quality can be solved.

Now, the process for fabricating this liquid crystal display isdescribed below. The difference from Example 1 lies in the fact that anITO layer is additionally formed on the reflector layer of the counterelectrode board of Example 1.

The method of making the TFT board is the same as employed in Example 1.

The method of making the counter electrode board is explained withreference to FIG. 5. First of all, a glass substrate 1 is provided asshown in FIG. 5(a) and a film of aluminum or silver having highreflectivity is deposited thereon by means of a sputtering apparatus asshown in FIG. 5(b). A resist is applied thereto, exposed to light withan exposure apparatus through a mask having a pattern in which the partscorresponding to the signal lines patterned on the TFT board are removedin the form of slits, and then developed to leave the resist having theaforesaid pattern. Next, the metal film is etched by using the patternedresist as a mask to remove the parts of the metal film which are notcovered with the mask. Thereafter, the resist is stripped off as shownin FIG. 5(c). Subsequently, an ITO layer 2 is deposited on the wholesurface of glass substrate 1 so as to cover reflector layer 6 as shownin FIG. 5(d). Finally, a polyimide is applied with a printer and thenrubbed to complete the counter electrode board as shown in FIG. 5(e).

Then, the TFT board and the counter electrode board which have been madein the above-described manner are bonded with a predetermined gaptherebetween. First of all, a sealant is printed on the film-bearingsurface of the TFT board by means of a seal printer. In order to securea predetermined gap between the two boards, a gapping material inparticulate form is incorporated in the sealant. Then, the TFT board andthe counter electrode board are bonded with the sealant so that thefilm-bearing surfaces thereof face each other. In this step, the twoboards are disposed in such a way that, when viewed from the outside ofthe panel, the patterns of the signal lines on the TFT board are alignedwith the slits on the counter electrode board. Finally, using a liquidcrystal injector, a GH liquid crystal is injected into the gap betweenthe two boards so bonded. Thus, an active matrix-driven reflection typeliquid crystal display as illustrated in FIG. 4 is fabricated.

What is claimed is:
 1. A reflection type liquid crystal displaycomprising;an active matrix board comprising active matrix elements atintersections of signal lines and scanning lines, said signal linesformed of a transparent conducting film and said scanning lines formedof a metal, a counter electrode board having a reflector layer thereon,and a liquid crystal sealed between said active matrix board and saidcounter electrode board, wherein said transparent conducting film andsaid reflector layer are in parallel planes, and said reflector layer isnot formed in any area of said counter electrode board located directlyopposite said signal lines, including the areas of said intersections.2. A reflection type liquid crystal display as claimed in claim 1wherein a transparent conducting film is formed on the whole surface ofsaid counter electrode board so as to cover said reflector layer.
 3. Aliquid crystal display unit comprising;a thin-film transistor boardcomprising,a first glass substrate layer, a conducting film layerincluding a plurality of signal lines and pixel electrodes, and a firstinsulating layer, wherein said conducting layer is disposed between saidfirst glass substrate layer and said first insulating layer, a liquidcrystal layer, and a counter electrode board comprising,a second glasssubstrate layer, a reflective layer, and a second insulating layer,wherein said reflective layer is disposed between said second glasssubstrate layer and said second insulating layer and is not located atany area directly opposite said signal lines, including areas whereinsaid signal lines intersect with a plurality of scan lines.
 4. Theliquid crystal display unit disclosed in claim 3, wherein said counterelectrode board further comprises a transparent conducting layerdisposed between said reflective layer and said second insulating layer.